1. Field of the Invention
The present invention relates to a method and device for correcting a detection signal output from a solid-state detector, as well as a solid-state detector having a correcting capability for use therewith, and more specifically, to a device and a method for correcting an image signal or other signal which is output from such a solid-state detector as a solid-state image sensor, including a CCD image sensor which detects visible light and outputs an image signal, and a radiation solid-state detector which detects radiation and outputs an image signal, and a solid-state detector having this correcting capability.
2. Description of the Prior Art
Up to now, solid-state image sensors such as a CCD image sensor, which detects visible light and outputs an image signal, have been widely used in such applications as video cameras and digital still cameras. This solid-state image sensor comprises a number of photoelectric transducers disposed in the form of a matrix (for color applications, a color filter is further overlaid upon each photoelectric transducer) outputting an image signal (consisting of pixel signals each representing the signal value of each pixel) carrying a visible image as two-dimensional matrix information.
Nowadays, in the field of radiation photography for medical diagnosis, a variety of radiation solid-state detectors (mainly consisting of a semiconductor, and hereafter may be simply called xe2x80x9cdetectorsxe2x80x9d), which detect radiation in a form of latent electric charges of an amount corresponding to the dose of the radiation to which the detectors have been exposed and record the radiation image in a form of an electrostatic latent image and output an image signal carrying the recorded electrostatic latent image, have been proposed and put to practical use. As a typical one of the various types of radiation solid-state detectors proposed, the radiation solid-state detector of photoelectric conversion type, which reads out the stored charges (also called the xe2x80x9clatent image chargesxe2x80x9d) carrying image information by means of thin film transistors (TFTs), a direct conversion type and an improved direct conversion type, a mode of the direct conversion type (also called xe2x80x9clight reading typexe2x80x9d) wherein the reading light is projected for scanning and reading out the latent image charges, are available. These types will be explained in the section titled xe2x80x9cDESCRIPTION OF THE PREFERRED EMBODIMENTSxe2x80x9d.
With any one of the above-mentioned various types of radiation solid-state detectors, the solid-state detecting elements are disposed in the form of a matrix, and the output is an image signal (consisting of pixel signals each representing the signal value of each pixel) representing a radiation image as two-dimensional information.
Hereinbelow, a solid-state image sensor which detects visible light and outputs an image signal representing a visible image as two-dimensional matrix information and a radiation solid-state detector which detects radiation and outputs an image signal representing a radiation image as two-dimensional matrix information are collectively referred to as xe2x80x9csolid-state image detectors.xe2x80x9d When a solid-state image detector can output not only two-dimensional, but also one-dimensional information, it is referred to as a xe2x80x9csolid-state detectorxe2x80x9d . A variety of elements, such as the photoelectric transducer constituting a solid-state image sensor and the solid-state detecting element constituting a radiation solid-state detector (described later) are collectively called xe2x80x9cdetecting elementsxe2x80x9d.
With the detecting elements constituting a solid-state image detector as stated above, the characteristic of quantity of incident light or dose of incident radiation versus output signal value (hereafter called the xe2x80x9cinput-output characteristicxe2x80x9d) varies from element to element, and if uniform radiation or light (hereafter generically called xe2x80x9cuniform radiationxe2x80x9d) is projected on the entire surface of the solid-state image detector, the image signals output from the detecting elements constituting the solid-state image detector will have variations.
The variations in input-output characteristic results from various factors, such as the variations in the sensitivity of the detecting elements, variations in load capacity of the detecting elements, and variations in the gain and offset voltages of the output amplifiers connected to the detecting elements to output the detected image signals. Also, these variations cause the image signals to have noise, and if image output is carried out on the basis of the image signals having such variations, the output image will include noise and have a deteriorated image quality.
To correct these variations of the image signals, methods for correcting the image signals output from a solid-state image detector have been proposed (for example, Japanese Unexamined Patent Publication No. 7 (1995)-72256).
With this image signal correcting method, the correction is made for each of the detecting elements (the solid-state light detecting elements) constituting a radiation solid-state detector (or for each group of elements comprising a set number of detecting elements) so that the values of the image signals when radiation is not projected (hereafter called xe2x80x9cin the dark statexe2x80x9d) is nullified. When the correction values for the image signals when uniform radiation is projected so that the detecting elements are irradiated with an equal dose of radiation (hereafter called xe2x80x9cin the bright statexe2x80x9d) that is approximately the same for all the detecting elements (or the groups of elements) are determined, the output image signals from the radiation detector are corrected based on these correction values. Also, the offset correction values, for correcting so that the values of the image signals in the dark state are nullified, and the gain correction values, for correcting so that the image signals in the bright state are approximately the same for all the detecting elements (or the groups of elements), are used as the correction values in the correction. Thus, this method to be used suppresses the noise which would be included in the image signals, allowing a high-quality radiation image to be output.
However, with the above-mentioned signal correcting method, the specification only states that the correction is made so that the values after the correction in the bright state are roughly uniform for all the detecting elements (or the groups of elements). The specific value that is to be selected is not stated. Depending upon the value, a problem may occur in that, when radiation having a dose of radiation that would saturate the detecting elements is projected onto all of the detecting elements, the value for one pixel is transformed into a maximum value which can be taken after the correction, while that for another pixel is transformed into a value less than the maximum, resulting in the image signals after the correction having variations. In other words, the correction made is insufficient.
For example, assume that the image signal of a detecting element xe2x80x9caxe2x80x9d, which has an image signal value of 50 in the dark state and an image signal value of 800 in the bright state, and the image signal of a detecting element xe2x80x9cbxe2x80x9d, which has an image signal value of 30 in the dark state and an image signal value of 900 in the bright state, are corrected in accordance with the correcting method of the reference cited. It is also assumed that the saturation values of either of the output image signals from the detecting elements xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d is 1000, and the maximum value which can be taken after the correction is also 1000.
First, the offset correction is made so that either of the image signal values in the dark state is nullified. On the other hand, it is assumed that the gain correction is made so that both of the values after the correction in the bright state are 800, in other words, the image signal of the detecting element xe2x80x9caxe2x80x9d is transformed from 800 to 800, and the image signal of the detecting element xe2x80x9cbxe2x80x9d is transformed from 900 to 800.
When radiation, having a dose of radiation at a level at which the output image signal from the detecting means xe2x80x9caxe2x80x9d or xe2x80x9cbxe2x80x9d is saturated, is projected, the image signal of the detecting means xe2x80x9caxe2x80x9d has a value of 1000, which is the saturation value for that image signal, and the value after the correction is also 1000, which is its maximum value after the correction. On the other hand, the image signal of the detecting means xe2x80x9cbxe2x80x9d also has a value of 1000, which is the saturation value for that image signal, but the value after the correction is approximately 900. Thus, the image signal values after the correction have a variation. If image output is carried out on the basis of the image signals after the correction having variations such as this, a problem exists in that the signal for one pixel is saturated while that for another pixel is not saturated. In other words, pixels whose signals are not saturated appear as granular noise, resulting in a difficult-to-view image.
The first pixel signal correction method according to the invention is a pixel signal correction method for correcting output pixel signals from a solid-state detector which detects visible light or radiation and obtains pixel signals each representing a signal value of a pixel, wherein said correction is made so that, when light or radiation at which one of the output pixel signals is at the saturation level is projected onto said solid-state detector, all the pixel signals are at a maximum value which can be taken as the signal value.
Here, the xe2x80x9csolid-state detectorxe2x80x9d is a detector having a number of solid-state detecting elements (corresponding to pixels) mainly composed of semiconductor elements which detect visible light or radiation, mainly consisting of a semiconductor, and is exemplified by the above-mentioned photoelectric transducer and solid-state detecting element. The solid-state detecting element is a detector having a number of detecting elements as defined above, and is exemplified by the above mentioned solid-state image sensor and radiation solid-state detector. This solid-state detector may be in the one dimensional form or the two-dimensional form. This is the same hereafter.
The second pixel signal correction method according to the invention is a pixel signal correction method for correcting output pixel signals from a solid-state detector which detects visible light or radiation and obtains pixel signals each representing a signal value of a pixel, wherein the greatest pixel signal value of the pixel signals when light or radiation, at which any one of the pixel signals of said detector is at a level lower than the saturation level, is projected onto said solid-state detector is determined, said correction is made for each of said pixel signals so that the signal value of each pixel exceeds said greatest pixel signal value.
Here, xe2x80x9cthe greatest pixel signal valuexe2x80x9d means the greatest pixel signal value when the input-output characteristic of the detecting elements has a positive characteristic (the pixel signal value is increased as the quantity of light or the dose of radiation is increased). When the input-output characteristic is negative (the pixel signal value is decreased as the quantity of light or the dose of radiation is increased), the phrase xe2x80x9cthe greatest pixel signal valuexe2x80x9d should be replaced with the phrase xe2x80x9cthe smallest pixel signal valuexe2x80x9d and the phrase xe2x80x9cfor each of the pixel signals, the correction is made so that the signal value exceeds the greatest pixel signal valuexe2x80x9d as given in the above paragraph should be replaced with the phrase xe2x80x9cfor each of the pixel signals, the correction is made so that the signal value is less than the smallest pixel signal valuexe2x80x9d. Thus, through the replacement of the pertinent phrases and statements according to the input-output characteristic, the specification of the present invention covers not only application where the input-output characteristic of the detecting elements is positive, but also an application where the input-output characteristics is negative.
The first pixel signal correction device according to the present invention is a device which realizes the above-stated first pixel signal correction method, i. e., a pixel signal correction device which corrects output pixel signals from a solid-state detector which detects visible light or radiation and obtains pixel signals each representing a signal value of a pixel comprising:
irradiating means which irradiates said solid-state detector with light or radiation at a level at which one of the pixel signals of said detector is at the saturation level; and
correcting means which makes said correction so that all the pixel signal values, in the state where light or irradiation is provided at which anyone of said pixel signals reaches the saturated level, are at a maximum value which can be taken as the signal value.
The second pixel signal correction device according to the present invention is a device which realizes the above-stated second detection signal correction method, i. e., a pixel signal correction device which corrects output pixel signals from a solid-state detector which detects visible light or radiation and obtains pixel signals each representing a signal value of a pixel comprising:
irradiating means which irradiates said solid-state detector with light or radiation at a level at which anyone of the pixel signals is at a level lower than the saturation level; and
correcting means which determines the greatest pixel signal value in the state where light or irradiation is provided at a level lower than said saturation level, said correction is made for each of the pixel signals so that the signal value exceeds said greatest pixel signal value.
As the solid-state detector for use in the above-mentioned pixel signal correction method and apparatus, various types of detectors can be used. For example, it is possible to use a light read-out type radiation solid-state detector comprising a first electrode layer, a photoconductive recording layer which shows conductivity upon exposure to recording light, a photoconductive read-out layer which shows conductivity upon exposure to reading light, and a second electrode layer provided with a stripe electrode consisting of a number of linear electrodes.
Further, it is possible to use a solid-state detector as disclosed in Japanese Patent Application No. 11 (1999)-87923, i. e. a solid-state detector comprising a first electrode layer provided with a first stripe electrode consisting of a number of linear electrodes, a photoconductive recording layer which shows conductivity upon its exposure to recording light, an electric storing portion for storing electric charges generated in said photoconductive recording layer, a photoconductive pre-exposure layer which shows conductivity upon its exposure to pre-exposure irradiation for uniformly charging said storing portion, and a second electrode layer provided with a second stripe electrode consisting of a number of linear electrodes arranged to cross with said linear electrodes of said first stripe electrode, said layers and portion being disposed in the above order.
It is further possible to use a solid-state detector as also disclosed in Japanese Patent Application No. 11 (1999)-87923, i. e. a solid-state detector comprising a first electrode layer provided with a first stripe electrode consisting of a number of linear electrodes, a photoconductive recording layer which shows conductivity upon its exposure to pre-exposure light and recording light, an electric storing portion for storing electric charges generated in said photoconductive recording layer, a dielectric layer, and a second electrode layer provided with a second stripe electrode consisting of a number of linear electrodes arranged to cross with said linear electrodes of said first stripe electrode, said layers and portion being disposed in the above order.
It is also possible to use a solid-state detector as also disclosed in Japanese Patent Application No. 11 (1999)-232763, i. e. a solid-state detector comprising a first electrode layer provided with a first stripe electrode consisting of a number of linear electrodes, a photoconductive recording layer which shows conductivity upon its exposure to recording light, an electric storing portion for storing electric charges generated in said photoconductive recording layer, a rectifying layer, and a second electrode layer provided with a second stripe electrode consisting of a number of linear electrodes arranged to cross with said linear electrodes of said first stripe electrode, said layers and portion being disposed in the above order.
Here, the xe2x80x9crecording lightxe2x80x9d means not only an electromagnetic wave emitted directly from a source of light or radiation (for example, visible light), but also an electromagnetic wave emitted from a phosphor or the like stimulated by such electromagnetic wave (for example, visible light) having a different wavelength from that of such electromagnetic wave.
The first solid-state detector according to the present invention is a solid-state detector having the above-stated first pixel signal correcting capability, i. e., a solid-state detector which detects visible light or radiation and obtains pixel signals each representing a signal value of a pixel comprising:
correcting means which corrects the output pixel signals from said detector so that all the pixel signal values when light or radiation at a level at which one of the pixel signals is at the saturation level is projected on said detector, are at a maximum which can be taken as the signal value.
The second solid-state detector according to the present invention is a solid-state detector having the above-stated second pixel signal correcting capability, i. e., a solid-state detector which detects visible light or radiation and obtains pixel signals each representing a signal value of a pixel comprising:
correcting means which determines the greatest pixel signal value when light or radiation at a level at which any one of the output pixel signals of said detecting elements is at a level lower than the saturation level is projected on said detector, and corrects the output pixel signals from said detector so that the signal value of each of the pixel signals exceeds said greatest pixel signal value.
With the first pixel signal correcting method and device according to the present invention, as well as the solid-state detector having the correcting capability, the correction is made so that, when light or radiation at a level at which any one of the pixel signals is at the saturation level (hereafter called light having xe2x80x9ca maximum quantity of lightxe2x80x9d or radiation having xe2x80x9ca maximum dose of radiationxe2x80x9d) is projected onto the detector, all the signal values of the output signals are at a maximum value which can be taken as the signal value. Therefore, if the detecting elements have variations in input-output characteristic, the values of the signals (pixel signal), when photographing is carried out under the condition that for one of the detecting elements a maximum quantity of light or a maximum dose of radiation is provided, are transformed after the correction into a maximum value that can always be taken by the signals after the correction (pixel signal after correction) of all the detecting elements. Thus, the image signals when photographing is carried out with a maximum quantity of light or a maximum dose of radiation are free from variations, which allows a high-quality image to be offered.
With the second detection signal correcting method and device according to the present invention, as well as the solid-state detector having the correcting capability, the greatest output signal value of the values of the output signals, when light or radiation at a level at which anyone of the detection signals is at a level lower than the saturation level is projected onto the detecting means of the solid-state detector, is determined, and for each of the output signals, the correction is made so that the signal value exceeds the greatest output signal value. Therefore, as long as the dynamic ranges for the image signals are approximately the same (later described in detail), as stated above, the values of the signals of the detection elements, when photographing is carried out under the condition that for one of the detecting elements a maximum quantity of light or a maximum dose of radiation is provided, are transformed after the correction into a maximum value which can be taken by the signals after the correction of all the detecting elements without fail. Thus, the image signals when photographing is carried out with a maximum quantity of light or a maximum dose of radiation are free from variations, which allows a high-quality image to be offered.
The purpose of the present invention is to provide a detection signal correcting method and a detection signal correcting device wherein, in correcting the variations in input-output characteristic of the detecting elements, the correction is made so that, when light having a quantity or radiation having a dosage at a level at which the detection signals output from the detecting elements are saturated is projected, the signals after the correction have no variations, as well as a solid-state detector which can realize the detection signal correcting method (hereafter referred to as xe2x80x9chaving a correcting capabilityxe2x80x9d).